QUANTIFICATION OF SYMBIOTICALLY FIXED NITROGEN IN SOIL SURROUNDING ALFALFA ROOTS AND NODULES

Symbiotically fixed nitrogen (SFN) can be transferred from alfalfa (Medicago sativa L.) to intercropped nonlegumes and succeeding crops. Our objective was to quantify deposition of SFN from alfalfa roots and nodules to surrounding soil and to assess its significance in comparison to reported estimates of legume-nonlegume N transfer. We quantified net loss of SFN to rhizosphere soil (i.e. rhizodeposition) at the end of one growing season under field and greenhouse conditions, using the N-15 isotope dilution technique in soil that had been labeled with N-15 8 yr before initiation of the experiments. 'Saranac' and 'In-effective Saranac' alfalfa were transplanted in the field into paired 0.75- by 1.0-m microplots. In the greenhouse study these two alfalfas were grown separately in 15-cm diam. by 90-cm long polyvinylchloride tubes, four plants per tube. Plant organs were meticulously separated from soil at the end of one growing season. Soil adhering to roots and nodules was defined as rhizosphere and nodusphere soil, respectively, and was separated from the organs by 60 s sonication in 0.08 mol L-1 phosphate buffer. Symbiotically fixed N was quantified by comparing the atom % N-15 of Saranac vs. Ineffective Saranac for each soil and plant N compartment. Harvested herbage contained 270 kg SFN ha-1 in the 135-d field experiment and 55 mg SFN plant-1 in the 112-d greenhouse experiment. Symbiotically fixed N was not uniformly distributed in the Saranac root system. Sampled rhizosphere soil contained no SFN in the field experiment and the equivalent of about 1 kg SFN ha-1 in the greenhouse experiment. Over one quarter of nodusphere soil N was apparently derived from SFN in both experiments, but amounted to less than 0.5 kg SFN ha-1 at the time of harvest Nitrogen transfer to nonlegumes is often an order of magnitude larger than net deposition measured here. Thus it appears that rhizodeposition is a relatively small component of SFN loss from nodules and roots of alfalfa grown in monoculture.